The present invention relates to a laser scanning device that forms images by scanning a laser beam, and more particularly to a laser scanning device of which laser beam power is adjustable.
A laser scanning device forms an image on a photo-conductive drum by scanning thereon a laser beam that is emitted from a laser source such as a semiconductor laser. If a gray-scale black and white image or a color image is to be produced, the laser scanning device is required to control the power of the laser beam in accordance with the shades of gray or the color to be printed. In order to achieve the above, the laser scanning device is provided with a laser power control circuitry which controls the power of the laser beam in accordance with a laser power control signal provided from an external device.
FIG. 1 is a block diagram of an exemplary laser power control circuitry 1000 utilized in a conventional laser scanning device. The laser power control circuitry 1000 is connected to a semiconductor laser LD such as a laser diode, and a photo-diode MPD for monitoring the power of the laser beam emitted from the semiconductor laser LD. The laser power control circuitry 1000 includes a laser driving circuitry 1010 that controls the laser beam emission of the semiconductor laser LD. The laser driving circuitry 1010 is arranged to receive a video signal to control the ON/OFF of the semiconductor laser LD in accordance therewith. The laser driving circuitry 1010 supplies a driving current Id to the semiconductor laser LD so that the semiconductor laser LD emits the laser beam. The laser driving circuitry 1010 controls the amount of the driving current Id in accordance with a reference voltage Vref provided from a reference voltage controller 1020. The reference voltage controller 1020 generates the reference voltage Vref in accordance with a laser power control signal provided from an external device.
The external device providing the laser power control signal may be, if the laser power control circuitry 1000 is applied to a laser printer, a print engine of the laser printer. The laser power control signal is, for example, a parallel four bits digital signal D3-D0, which is provided to the reference voltage controller 1020 through four input terminals Td3-Td0 that are pulled up to a source voltage Vcc by respective pull-up resistors R. The reference voltage controller 1020 generates the reference voltage Vref, for example, by converting the digital laser power control signal into an analog signal and outputs the same to the laser driving circuitry 1010. The laser driving circuitry 1010 controls the driving current Id supplied to the semiconductor laser LD based on the reference voltage Vref.
The semiconductor laser LD supplied with the driving current Id emits the laser beam, and the power of this laser beam is detected by the photo-diode MPD. The photo-diode MPD outputs a laser power detection current Ir corresponding to the detected laser beam power to the laser driving circuitry 1010. The laser power detection current Ir is converted into a laser power detection voltage Vr within the laser driving circuitry 1010. The laser driving circuitry 1010 compares the laser power detection voltage Vr and the reference voltage Vref and adjusts the driving current Id supplied to the semiconductor laser LD so that laser power detection voltage Vr and the reference voltage Vref indicates a predetermined relation, for example, coinciding with each other.
The laser driving circuitry 1010 is further provided with a variable resistor VR for varying the rate of converting the laser power detection current Ir into the laser power detection voltage Vr. By properly adjusting the variable resistor VR, the power of the laser beam emitted from the semiconductor laser LD corresponds properly to the reference voltage Vref, or the laser power control signal provided from the external device.
In the laser scanning device provided with the laser power control circuitry 1000, the reference voltage Vref for controlling the driving current Id of the semiconductor laser LD is generated from the laser power control signal provided from the external device.
An adjustment to the laser power control circuitry 1000 is carried out in the following manner. First, the laser power control circuitry 1000 is connected with an external adjustment device that provides a standard laser power control signal to the reference voltage controller 1020. The reference voltage controller 1020 generates a reference voltage Vref corresponding to the standard laser power control signal. Next, the laser driving circuitry 1010 provides a driving current Id corresponding to that reference voltage Vref to the semiconductor laser LD. As a result, the semiconductor laser LD emits a laser beam. The power of the laser beam is adjusted so as to correspond to the standard laser power control signal by operating the variable resistor VR.
It should be noted, however, that if the laser power control circuitry 1000 is (or the data input terminals Td3-Td0 thereof are) poorly connected with the external adjustment device or with a signal bus connecting the laser power control circuitry 1000 and the external adjustment device, the laser power control circuitry 1000 receives the laser power control signal erroneously, and hence generates the reference voltage Vref and controls the driving current Id based on that erroneous laser power control signal.
Assume, for example, that a laser power control signal indicating a maximum value “1111” is provided to the reference voltage controller 20 through the data input terminals Td3-Td0, and the variable resistor VR is adjusted so that the semiconductor laser LD outputs the maximum power thereof. If one of the data input terminals, for example, the data input terminal for the most significant bit (MSB), Td3, is poorly connected with external adjustment device, the laser power control circuitry 1000, or the reference voltage controller 1020, receives an erroneous data “0111”. In this case, the laser power control circuit 1000 is adjusted to control the semiconductor laser LD to output the maximum power although the provided laser power control signal is not the maximum value thereof.
If the laser scanning device of which laser power control device 1000 is adjusted as above is provided in a laser beam printer, and if a laser power control signal having the maximum value “1111” is inputted to the laser power control device 1000, the semiconductor laser LD is supplied with a driving current Id exceeding the maximum allowable driving current (in the current case, with a driving current twice as large as the maximum allowable driving current). As a result, the semiconductor laser LD outputs excessive power, which may accelerate the deterioration and/or cause fatal damage to the semiconductor laser LD.
Therefore, there is a need for a laser scanning device by which the laser beam power can be adjusted correctly and reliably.